The present application is a United States National Stage application based on International Application No. PCT/SE99/01391, filed on Aug. 18, 1999, entitled xe2x80x9cEntraining Device for a Centrifugal Separatorxe2x80x9d, and claiming priority to Swedish Patent Application No. 9802784-0, filed on Aug. 20, 1998. Both PCT/SE99/01391 and Swedish Patent Application No. 9802784-0 are incorporated by reference herein.
The present invention concerns an entraining device for a centrifugal separator having a rotor rotatable around a rotational axis. The rotor forms an inlet chamber in which an inlet tube opens for the supply during operation of a mixture of components to be separated. The inlet chamber has an outlet opening at a certain axial position in the inlet chamber. The rotor also forms a separation chamber that communicates with the outlet opening of the inlet chamber via at least one flow channel but otherwise is separated from the inlet chamber by a partition wall, which surrounds the rotational axis and has an axial extension the inside of which delimits the inlet chamber radially outwardly. Furthermore, the rotor forms at least one outlet for a component of the mixture separated during operation.
The entraining device is arranged in the inlet chamber fixedly connected, directly or indirectly, to the rotor. The entraining device extends axially along substantially all the axial length of the inlet chamber and comprises at least one liquid conducting element having two axial delimiting surfaces. One axial delimiting surface turns axially towards and the other turns axially away from the inlet chamber outlet. The entraining device extends radially and circumferentially in the inlet chamber and is at least partly located in a portion of the inlet chamber that during operation is filled with the mixture. The entraining device has a radial inner edge over which the mixture can flow during operation when the mixture level in the inlet chamber is located radially inside this edge and at least one flow passage arranged in the inlet chamber nearby the radial inside of the partition wall.
In each one of the following U.S. Pat. No. 4,701,158, U.S. Pat. No. 4,721,505 and WO-A-95/12082 there is disclosed a centrifugal separator, that has an entraining device. The entraining device is of the kind in which the inlet chamber is in the form of a number of discs that surrounds the rotational axis. The discs extend radially and circumferentially defining between themselves interspaces through which the mixture flows radially outwardly.
Due to the fact that the entraining devices in these centrifugal separators have large contact surfaces, which during operation entrain the supplied liquid mixture into the rotation of the rotor and which extend radially and in the circumferential direction, the entrainment takes place gently along these large surfaces. The higher the flow of the mixture supplied to these centrifugal separators, the more discs that attend automatically to the increased need of entrainment, as the mixture overflows the radial inner edges of even more discs. However, when the flow of the mixture is low, the mixture does not flow radially outwardly in all interspaces, which means that there is no axial flow along a portion of the insides of the partition walls, which delimits the inlet chambers radially outwardly towards the separation chambers.
In many cases, this means that sludge particles are deposited on the inside of the partition walls. Since there is no space in these centrifugal separators to design the inside of the partition walls with such a large angle relative to the rotational axis, these sludge particles can slip due to the centrifugal force along the inside of the partition walls towards the outlet openings of the inlet chambers. When this occurs the sludge particles will accumulate on of the inside of the partition walls. If this is allowed to continue, the inlet chambers eventually will become clogged. When clogging occurs, the centrifugal separation has to be interrupted for cleaning of the centrifugal separator.
In DE-C-30 41 210 and WO-A-97/17139, proposals are disclosed for cleaning the interior of the centrifugal separator. However, in the two proposals, the centrifugal separation has to be interrupted and valuable production time is lost. In many cases, you cannot get the centrifugal separator clean enough by the proposed methods, and the centrifugal separator still has to be disassembled, cleaned and re-assembled, which is a very labor intensive and time-consuming operation.
The object of the present invention is to design an entraining device for a centrifugal separator, which entrains the mixture, and which makes it possible to operate the centrifugal separator during long periods of time without the inlet chamber becoming clogged.
According to the present invention, the above-described object is in one aspect accomplished by one of the delimiting surfaces turning away from the outlet opening of the inlet chamber. As stated earlier, the delimiting surface turning away from the outlet opening of the inlet chamber comprises a surface portion, which during operation is at least partly located in a part of the inlet chamber that is filled with a mixture, and which in the circumferential direction extends axially in such a way that the delimiting surface seen in the rotational direction extends towards the outlet opening of the inlet chamber. In other words, the delimiting surface turning away from the outlet opening of the inlet chamber has a normalcy that has a component in the rotational direction.
In one embodiment of the invention, the axial extension in the circumferential direction is more than 0.5 mm but less than 100 mm.
In another embodiment of the invention, the surface portion consists of the entire delimiting surface, which is turned axially away from the outlet opening of the inlet chamber and that the axial extension of it in the circumferential direction is the same all along this delimiting surface.
In a further embodiment of the invention the two delimiting surfaces are substantially planar. Suitably, the two delimiting surfaces are parallel.
In a preferred embodiment of the invention, the delimiting surfaces extend in the circumferential direction in a helically shaped path at least one revolution around the rotational axis.
Preferably, the flow passage is annular surrounding the rotational axis.
In still another embodiment of the invention the inlet chamber has an axial end, in which the inlet tube opens and in which the outlet opening is located.